Soundcraft Venue II sum amp problem

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syn

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Just for the record, I pulled the master of my Venue today and found in the left summing amp position a NE5532 and in the right position a TL072. Didn´t notice it for a decade....
That enhances the stereo image
 

JohnRoberts

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Just for the record, I pulled the master of my Venue today and found in the left summing amp position a NE5532 and in the right position a TL072. Didn´t notice it for a decade....
As Abbey suggested not being immediately apparent suggests noise sources elsewhere could dominate the noise floor.

The other difference between those two is slightly more open loop gain at 20 kHz for the 5532, but that will mainly express as more phase shift and THD at very HF (in 072).

If not mixing a huge number of stems, unlikely to be audible.

JR
 

syn

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OK, this is getting strange... Up to today I haven't been paying much attention to master fader noise that I mentioned earlier.
Actually it sounds more like stutter, not noise, this board is very quiet apart from this anomaly.
I found some time today to play more with my master module and regardless of IC1 IC2 and IC3 model ( tested with 072, 5532, 49720, 1612, 1656, 4562, 33078) I have a strange stuttering effect when I pull my master faders up or down. Cleaning faders and even replacing them, made no difference.

There is no DC on the faders. 0VDC.
C6-C106 fitted polyprop caps.

This is what 072 shows in position IC1 and IC 2 --- IC3 is 5532

IC1
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+/-16.5VDC [email protected] pins 8&4

IC2
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+/-16.5VDC [email protected] pins 8&4

There is no DC on IC3 apart from +/-16.5VDC supply

Please listen to the attached wav. @~ 5th second I pull the master faders down and than fade in again.

Any ideas?
 

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syn

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I repeated measurement, this time with 1612 in all 3 positions IC1, 2 & 3. VDC readings are exactly the same as with 072, that I posted a minute ago.
 

syn

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Very, very slightly for example +6.93VDC with fader pulled down becomes +6.89V or so. The largest swing is on pin7 --- IC 1:
fader all the way up -1.309V, mid position -1.316V and pulled all the way down -1.296V.

Thank you
 

abbey road d enfer

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Very, very slightly for example +6.93VDC with fader pulled down becomes +6.89V or so. The largest swing is on pin7 --- IC 1:
fader all the way up -1.309V, mid position -1.316V and pulled all the way down -1.296V.
Weird. The fader being located after, it shouldn't have any effect there.
Are you positive there is no oscillation?
 

syn

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As far as I can see on my little scope, no oscillation. TL072 IC1 and IC2 are about room temp. hot. the same goes for IC3 ne5532.
 

syn

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What is the bandwidth of your scope ? And sampling rate if digital.
Specifications:
- Maximum Real-time Sampling Rate: 1Msps
- Analog Bandwidth: 0-200KHz


and

master faders noise/stutter SOLVED, I cross wired faders send & return, now everything works as it should.

I tested the whole thing once more with hi speed opamps, and all good, smooth noiseless operation. At this point I'm convinced that OPA1612 hotter than I like operation is due to parts small size and heat dissipation mistreated on my part . I will install heat sinks on all 1612s and give them a chance. To my ears, they sound light years ahead of 072-5532 combo.

Thank you all so much
 
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abbey road d enfer

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At this point I'm convinced that OPA1612 hotter than I like operation is due to parts small size and heat dissipation mistreated on my part . I will install heat sinks on all 1612s and give them a chance. To my ears, they sound light years ahead of 072-5532 combo.
Why don't you try the LME49720? It is available in PDIP, and the performance has nothing to envy to the 1656.
 

syn

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Why don't you try the LME49720? It is available in PDIP, and the performance has nothing to envy to the 1656.
I did. It sounds better than 5532 IMO but still not as good as 1612, also 1656 is very good prefer it to 49720, but to my ears 1612 is a king. It has sound on its own. It adds depth and detail without being clinical and avoids that mid range "hole" that many other opamps have. I'm aware that all this is subjective and not everyone's preference.
 
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Matt Syson

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Going back to the original schematic, although one half (the 'B' section of the VE amplifier has a 75 Ohm resistor for 'isolation' (good), if you hang a long wire onto the insert send you start to curtail the HF negative feedback that goes back to the bus in the 'A' section of that chip. I suppose having the feedback invertor hanging from after the 75 Ohm resistor probably makes the situation worse. To be more thorough the 'isolation' resistor (or use small choke/resistor combination) should have been fitted in series with the insert send 'hot' to make it less sensitive to output loading
 

abbey road d enfer

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Going back to the original schematic, although one half (the 'B' section of the VE amplifier has a 75 Ohm resistor for 'isolation' (good), if you hang a long wire onto the insert send you start to curtail the HF negative feedback that goes back to the bus in the 'A' section of that chip. I suppose having the feedback invertor hanging from after the 75 Ohm resistor probably makes the situation worse.
Don't forget at the same time it reduces the OLG of the first opamp, so the overall stability is more or less intact.
 
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